Microbiology, 2021

688 17 • Innate Nonspecific Host Defenses
Figure 17.5
This scanning electron micrograph shows ciliated and nonciliated epithelial cells from the human trachea. The mucociliary
escalator pushes mucus away from the lungs, along with any debris or microorganisms that may be trapped in the sticky mucus, and the
mucus moves up to the esophagus where it can be removed by swallowing.
The mucociliary escalator is such an effective barrier to microbes that the lungs, the lowermost (and most
sensitive) portion of the respiratory tract, were long considered to be a sterile environment in healthy
individuals. Only recently has research suggested that healthy lungs may have a small normal microbiota.
Disruption of the mucociliary escalator by the damaging effects of smoking or diseases such as cystic fibrosis
can lead to increased colonization of bacteria in the lower respiratory tract and frequent infections, which
highlights the importance of this physical barrier to host defenses.
Like the respiratory tract, the digestive tract is a portal of entry through which microbes enter the body, and
the mucous membranes lining the digestive tract provide a nonspecific physical barrier against ingested
microbes. The intestinal tract is lined with epithelial cells, interspersed with mucus-secreting goblet cells
(Figure 17.6). This mucus mixes with material received from the stomach, trapping foodborne microbes and
debris. The mechanical action of peristalsis, a series of muscular contractions in the digestive tract, moves the
sloughed mucus and other material through the intestines, rectum, and anus, excreting the material in feces.
Access for free at openstax.org.